The forerunner of today's general purpose corrosion resistant Ni--Cr--Mo alloys was developed and patented in the 1930's (U.S. Pat. No. 1,836,317) by Russell Franks, working at the time for a predecessor to the developer of the present invention. The commercial embodiment of this invention was marketed under the name Alloy C and included, besides chromium and molybdenum, smaller amounts of iron, the option of a tungsten addition, and minor additions of manganese, silicon, and vanadium to aid in manufacturing. Alloys within this compositional range were found to exhibit passive behavior in many oxidizing acids by virtue of the chromium addition. Also, they exhibited good resistance to many non-oxidizing acids by virtue of the enhancement of nickel's natural nobility by molybdenum and tungsten additions.
Over the years, several discoveries related to this alloy family or system have been made. First, it was identified that carbon and silicon are quite deleterious to the corrosion resistance of these alloys, because they promote the formation of carbides and intermetallic precipitates (such as mu-phase) at grain boundaries within the microstructure. At high carbon and/or silicon levels, these compounds can form upon cooling after annealing, or during elevated temperature excursions, such as those experienced by weld-heat-affected-zones. Since the formation of these compounds depletes the surrounding regions of chromium, molybdenum (and, if present, tungsten), those regions become much more prone to chemical attack, or become "sensitized". The compounds themselves can also be attacked preferentially. A key patent relating to low carbon and low silicon Ni--Cr--Mo alloys (U.S. Pat. No. 3,203,792) having improved thermal stability was issued in 1965. The commercial embodiment of that patent was developed and marketed as Alloy C-276 by the successor to the Haynes Stellite Company and is still the most widely used alloy of this family.
Even with low carbon and low silicon levels, the Ni--Cr--Mo alloys are metastable, i.e. in combination, the alloying elements exceed their equilibrium solubility limits and eventually cause microstructural changes in the products. Exposure of the alloys to the approximate temperature range of 1200.degree. F. to 1800.degree. F. (or about 650-1000.degree. C.)quickly induces metallurgical changes, in particular the precipitation of intemetallic compounds in the grain boundaries, which weaken the structure. To reduce further the tendency for deleterious compounds to form, a tungsten-free, low iron composition called Alloy C-4 was developed and patented (U.S. Pat. No. 4,080,201) by co-workers of the present inventor. This patent required a carefully controlled composition and also included small but important amounts of titanium to combine with any residual carbon and nitrogen. Similarly, U.S. Pat. No. 5,019,184 again teaches that low iron and low carbon plus some titanium reduces Mu phase formation by enhancing thermal stability in these alloys.
Another important discovery with regard to C-type alloys containing both molybdenum and tungsten was that optimum corrosion and pitting resistance is dependent upon certain important elemental ratios. It was discovered during the development of C-22 Alloy that the Mo:W ratio should lie between about 5:1 and 3:1 and that the ratio of 2.times.Cr: Mo+(0.5.times.W) should fall in the range of about 2.1 to 3.7. See U.S. Pat. No. 4,533,414, also assigned to the assignee of the present invention.
More recently, U.S. Pat. No. 4,906,437 disclosed the subtle effects of the deoxidizing elements aluminum, magnesium, and calcium if kept within certain narrow, specified ranges, with regard to hot workability and influence on corrosion performance. The base composition described in U.S. Pat. No. 4,906,437 is quite similar to that discovered in 1964 by R. B. Leonard who, at that time, was researching C-type alloys for the assignee of the present invention. See G. B. Pat. No. 1,160,836. By performing potentiostatic studies on several compositional variants, Leonard identified Ni--23Cr--15Mo as a suitable design base for developing cast Ni--Cr--Mo alloys.
Of course, different families of alloys, containing some of the same elements but in differing proportions, have been developed to have different properties so as to satisfy different needs in the metallurgical arts. One example of such a different type of alloy is Alloy G, developed by the predecessor of the present assignee during the 1950's to resist phosphoric acid. It superficially resembles the C-type alloys except for containing much more iron and less molybdenum along with some cooper. It is more fully disclosed in U.S. Pat. No. 2,777,766.
Published information relating to the nominal compositions and corrosion properties of these prior art C-type alloys is summarized in Tables A and B.
The aforementioned patents are only representative of the many alloying situations reported to date in which many of the same elements are combined to achieve distinctly different functional relationships such that various phases form providing the alloy system with different physical and mechanical characteristics. Nevertheless, despite the large amount of data available concerning these types of nickel-base alloys, it is still not possible for workers in this art to predict with any degree of accuracy or confidence the physical and mechanical properties that will be displayed by certain concentrations of known elements even though such combinations may fall within broad, generalized teachings in the art, particularly when the new combinations may be thermo-mechanically processed somewhat differently from those alloys previously employed in the art.